POBox 52025 RECElVED Mail Station: PAB3S2 

Phoenix, AZ 85072-2025 Phone: (602) 236-2968 

(602) 236-5900 OCT 2 0 2010 Fax: (602) 236-6690 

wwwsrpnet.com Email: Kevin.Wanttaja@srpnet.com 

Permits Office Air-3 

October 15, 20 I0 U.S. EPA. Region 9 

Dr. Anita Lee 

U.S. Environmental Protection Agency Region IX 

75 Hawthorne Street 

San Francisco, CA 94105 

Re:	 Additional Information Requested During Meeting on September 28,2010 

Best Available Retrofit Technology Determination - Navajo Generating Station 

Dear Dr. Lee: 

Salt River Project (SRP) would like to thank United States Environmental Protection Agency (EPA), the 

National Park Service (NPS), and the United States Forest Service (USFS) for taking the time to meet 

with us on September 28, 20 IO. The purpose of the meeting was to discuss the results of SRP's ammonia 

monitoring program and the potential implications of the results on the visibility modeling analysis that 

will be performed as part of the pending Best Available Retrofit Technology (BART) proposal for the 

Navajo Generating Station (NGS). 

One of the topics discussed during this meeting was the modeling analysis (hereafter referred to as the 

"Benchmarking Analysis") that was included in Attachment 2 of SRP's comments on the Advanced 

Notice of Proposed Rulemaking (ANPR) issued by EPA in August 2009. In the ANPR, EPA used a 

back-calculation method to establish higher background ammonia concentrations than those that were 

used by SRP in the BART analysis submitted in January 2008. SRP performed the Benchmarking 

Analysis to determine which background ammonia inputs produce the most reasonable agreement 

between the model-predicted ammonium nitrate concentrations and the ammonium nitrate concentrations 

measured at sites that are part of the Interagency Monitoring of Protected Visual Environments 

(IMPROVE) program. The Benchmarking Analysis showed that the use of EPA's back-calculated 

ammonia concentrations produces order-of-magnitude overpredictions (i.e., by a factor of 10 or more in 

many cases) of ammonium nitrate concentrations relative to measured values. 

During the meeting on September 281", EPA requested additional information on the Benchmarking 

Analysis. The analysis compared model-predicted and measured results on specific days on which 

emissions from NGS were predicted by the model to reach the IMPROVE sites. EPA expressed concern 

that this part of the analysis might exclude days that would be relevant for consideration and that could 

change the conclusions of the Benchmarking Analysis. 

EC 14144,1010 
Page 2 

Dr. Anita Lee, U.S. EPA 

October 15, 2010 

In response to this question, several graphics are included in Attachment I of this letter that show how the 

days of interest were selected in the Benchmarking Analysis. A day was selected if the model-predicted 

ammonium sulfate concentration, which is insensitive to background ammonia, overpredicted or was a 

significant percentage of the measured value, and the concentrations were sufficiently high (well above 

zero) to merit consideration. On the selected days, since the model predicts emissions from NGS to reach 

the IMPROVE site, a comparison of model-predicted and measured ammonium nitrate values is 

appropriate and can be used to determine which set of ammonia inputs yields the best model performance. 

It is important to note that the analysis of specific days was only one of three approaches that were 

presented in the Benchmarking Analysis. The other two approaches considered illof the days throughout 

the 2001-2003 period. The same conclusion was reached using the other two approaches as in the dayspecific analysis: the use of EPA's back-calculated ammonia concentrations produces significant 

overpredictions of ammonium nitrate concentrations relative to measured values. 

SRP urges EPA to consider the following points as the agency develops a BART proposal for NGS: 

•	 The degree of visibility improvement predicted by the current regulatory CALPUFF model is 

critically dependent on the background ammonia concentration used as input to the model. 

Although the ranking of BART control options will not change as a result of changes to the 

ammonia concentration inputs to CALPUFF, the magnitude of the predicted visibility 

improvement from each BART control option will change substantially. 

•	 As discussed in the September 28th meeting, the current regulatory CALPUFF model has a 

general tendency to overpredict visibility impacts in winter. To compensate for this tendency, the 

visibility benefits modeling should be conducted using the background ammonia values that were 

used by SRP in the December 2008 BART analysis. 

•	 A sensitivity modeling analysis that uses EPA's back-calculated ammonia inputs would 

overestimate the degree of visibility improvement predicted for more stringent BART control 

options. It has been shown that the use of EPA's back-calculated ammonia inputs produces 

significant overestimates compared to measured values. 

•	 The nitrate formation modules in CALPUFF should be updated in order to correct the general 

tendency of the model to overpredict nitrate concentrations in the winter. To facilitate the 

CALPUFF model improvement effort, WEST Associates is sponsoring the incorporation of the 

nitrate chemistry improvements into the model. SRP intends to run the updated version of the 

model to provide additional information to EPA for use in the BART determination for NGS. 

•	 In the meantime, given the tendency of the currently approved version of the CALPUFF model to 

overpredict nitrate concentrations and the degree of visibility improvement associated with more 

stringent BART control options, SRP urges EPA to proceed with a BART determination that is 

based on SRP's ammonia inputs. 

EC 14144.1010 
Page 3 

Dr. Anita Lee, U.S. EPA 

October 15, 2010 

SPR appreciates EPA's consideration of this additional information. Please do not hesitate to call Grant 

Smedley at (602) 236-2928 if you have any questions. 

Sincerely, 

)""-- /I- 1.1 

Kevin Wanttaja 

Manager, Environmental Services 

Attachments 

EC 14144.1010 
Attachment 1: Selected Days for Benchmarking Analysis 
0 

0 

1/1/2001 

1/15/2001 

1/29/2001 

2/12/2001 

2/26/2001 

3/12/2001 

3/26/2001 

4/912001 

4/23/2001 

51712001 

5/21/2001 

6/412001 

6/18/2001 

7/212001 

7/16/2001 

7/30/2001 

8/13/2001 

8/27/2001 

9/10/2001 

9/24/2001 

10/8/2001 

10/22/2001 

11/512001 

11/19/2001 

12/312001 

12/17/2001 

AmmSulfate 24-hour Concentration (ugfm3 ) 

o N N 

a a (JI 

t +
 s:: \J (I) <D III Cl. tJ) 

c 

<D Cl. Cl. 

» »

3 3

3 3 

(/) (/) c c 

ro ro

c c 

(Q (Q 

3' 3' ~ ~

3 

Q)	 0 _::3 

OJ C' 

0<3

Ow 

CD	 c:: 

0:::;; 

Q)	 Q) 

,<CD ::3	 ­

00 

::3	 Q) 

5'0< 

~oQ)	 0 ..,	 ::3 

1\,)0

oCD

oa ..a.	 .., 

Q) 

!:!: 

o 

::3 

VI 
AmmSulfate 24-hour Concentration (ug/m3 ) 

0 0 ...... ...... IV 

0 (]'I 0 c.ro 0 

111/2002 

1115/2002 

1/29/2002 

2/12/2002 

2/26/2002 

3/12/2002 

3/26/2002 

4/912002 

4/23/2002 

5/7/2002 

5/21/2002 

6/412002 

6/18/2002 

71212002 

7116/2002 

7/30/2002 

8/13/2002 

8/27/2002 

9/10/2002 

9/24/2002 

10/8/2002 

10/22/2002 

11/5/2002 

11119/2002 

12/3/2002 

12/17/2002 

12/31/2002 

f +


s:: "U (\) 

III <il 

0­ C/l

c: n 

(\) <il 0­ 0­

3 3 

3 3

CIl CIl

c: c: 

(\) 

c C <0 <0 

*

3' 3' 

V> V> ....... .......
 

IV 

(]'I 

» 

3 

3 

D) 0 

- :!. llIe 

0.<3 

°cn (I) e o~D) D) 

,<(I) ~­

00 

D) 

5'0< 

-<0 

(I) 0 

D) 

.., 0 

N(I) 

o~0_ 

N~

o 

en 
AmmSulfate 24-hour Concentration (ug/m3 ) 

o o .... o (J'l o (J'l o'"

1::::::::~~::~_~:====~·~=;;;:::::·~-_--L_----_--l
 

1/29/2003 

2/26/2003 

3/12/2003 

6/18/2003L 

7/212003. -= 

7/16/2003" 

7/30/2003 " 

8/13/2003 j. 

8/27/2003~

- 9/10/2003,;[ .---­ 9/24/2003~ . ---* 

10/8/2003 'r-­

10/22/2003~ '-"; ... 

1115/2003 P.s 

. <;::;:::­

• 

• 

-

s 

. -::::::::::p,e, 

-,. 

-==-. 

t 

"'0 

+ CI> CD III 0­ m 0­ c 

CD r0­ o­ 0­

» » 

3 3 

3 3 

Ul Ul 

c c 

iif iif 

ro­ roc­ c­ co co 

3' 3' 

» 

3 

3 

Dl 0 .... ::::s

mc' 

-< 3 

(")CJ) 

(I) r:::: 

0­

Dlii1' 

::::s .... '< (I) 

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:;'-< 

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N(") 

0(1)

oa

WQl 

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0 

::::s 

t/) 

11/19/2003 
SELECTED DATES FOR BENCHMARKING ANALYSIS
 

BRYCE CANYON
 

Date 

10/16/200 I 

1/8/2002 

3/21/2002 

11/22/2002 

12/28/2002 

10/18/2003 

Measured
 

AmmSulfate
 

(ue:/m3) 

0.4 

0.19 

0.48 

0.22 

0.33 

0.75 

Predicted
 

AmmSulfate
 

(ue:lm3) 

0.402 

0.196 

0.784 0.521 0.404 

0.500 
Cl 

Cl 

1/1/2001 

Cl 

(Jl 

AmmSulfate 24-hour Concentration (ug/m3 ) 

!'> 

Cl 

..... ..... 6 c.r. N 

(Jl 

w 

Cl 

1/15/2001 

1/29/2001 

2/12/2001 

2/26/2001 

3/12/2001 

3/26/2001 

4/912001 

4/23/2001 

5/7/2001 

5/21/2001 

6/412001 

6/18/2001 

71212001 

7116/2001 

7/30/2001 

8/13/2001 

8/27/2001 

9/10/2001 

t 

s:: 

('I) 

III 

en 

c::

.., 

('I) 

0. 

» 

3 

3 

CIJ 

c:: 

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CD 

c 

co 

3" 

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('I) 

0. 

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('I) 

0. 

» 

3 

3 

CIJ 

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it 

CD 

c 

co 

3" 

I» 

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o 0 

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o _. I» r:::: 

3 3 

yenr:::: 

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(I) 

0.0 

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_.0 

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c;:~ I» I» 

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o~

ol/l 

...a. 

9/24/2001 

10/8/2001 

10/22/2001 

11/5/2001 

11/19/2001 

12/312001 

12/17/2001 

12/31/2001 
9 

0 

1/1/2002 

1/15/2002 

1/29/2002 

2/12/2002 

2/26/2002 

3/12/2002 

3/26/2002 

4/912002 

4/23/2002 

5/712002 

5/21/2002 

6/412002 

6/18/2002 

7/212002 

7/16/2002 

7/30/2002 

8/13/2002 

8/27/2002 

AmmSulfate 24-hour Concentration (ug/m3 ) 

N N w ()'1 6 u, o U, o 9 

t \)

+
 s:: .., CD CD III C. CIl 

t:

.., n 

CD CD 

C. c.

» »

3 3

3 3

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& 

t: 

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'2 '2

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3 

co co 

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3 

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(I) 

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c: 

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Q.C 

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-
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(I) 

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01/1 

N 
0 

0 

11112003 

1115/2003 

1/29/2003 

2/12/2003 

2/26/2003 

3/12/2003 

3/26/2003 

4/912003 

4/23/2003 

51712003 

7/2/2003 

9/24/2003 

10/8/2003 

10/22/2003 k 

11/5/2003 

11/19/2003 

12/312003 

12/17/2003 

AmmSulfate 24-hour Concentration (ug/m3 ) 

0 N N 

<n 0 <.n o <n 

..... --= ..
 

t +
 s: -0 

.... l1> l1> I\) 0­

c 

en n: 

l1> iil 0­ 0­

» »

3 

3 

3 3 

en en

c 

it 

c 

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ro ro

c C to to 

3" 3" 

w 

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-»

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(l) ::J o _. I» r::::: 

3 3 

-0 00

r::::: 

~Dr' I» ­ ::J (l) 

0.0 

o ~. I» ­ ::J'< 

,<0 

o 0 

::J ::J 

_.0 

::J (l) 

I» I» 

~~

.., 

~o'­0::J 

OUI 

W 
SELECTED DATES FOR BENCHMARKING ANALYSIS 

HANCE CAMP 

Measured Predicted 

Date AmmSulfate AmmSulfate 

(u2lm3 ) (u2lm3 ) 

12/24/2001 0.24 0.217 12/27/2001 0.13 0.424 

1/11/2002 0.24 0.218 

2/4/2002 0.41 0.314 

2/7/2002 0.22 0.333 

2/22/2002 0.19 0.149 

10/14/2002 0.98 0.930 

11/19/2002 0.41 0.373 

12/4/2002 0.48 0.354 1/18/2003 0.16 0.389 
0 

0 

1/1/2001 

0 

i:.n 

AmmSulfate 24-hour Concentration (ug/m 3 ) 

N 

o -" -" 

0 i:.n N 

(J'I 

w 

o 

1/15/2001 -

1/29/2001 

2/12/2001 

2/26/2001 

3/12/2001 

3/26/2001 

4/912001 

4/23/2001 

5n12001 

5/21/2001 

6/412001 

6/18/2001 

7/212001 

7/16/2001 

7/30/2001 

8/13/2001 

8/27/2001 

t 

(l) 

II> 

U> 

c.., 

(l) 

a. 

» 

3 

3 

(J) 

c 

iif 

eD 

c 

lC 

3' 

""U 

+

CD 

a. 

IT 

(l) 

a. 

» 

3 

3 

(J) 

c 

iif 

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c 

lC

-­3 

Dl

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-»

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;'3 

0 

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0. 3 

CD CIJ _~ C 

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Q. 

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(")::

Dl'< 

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o 

0 

-'CD 

~~

Dl~

~o' o~

ol/l 

..a. 

9/10/2001 

9/24/2001 

10/8/2001 

10/22/2001 

11/512001 

11/19/2001 

12/312001 
ZOOZ/6 ~/Z~

ZOOZ/g/Z~

ZOOWW~

zooz/m~

ZOOZlvZ/O~

ZOOZ/O ~/O~

ZOOZ/9Z/6 

ZOOZml6 

ZOOZ/6Z/9 

ZOOZ/g~/9

ZOOW/9 

ZOOZ/9~/L

ZOOZML 

ZOOZ/OZ/9 

ZOOZ/9/9 

ZOOZ/£Z/g 

ZOOZ/6/g 

ZOOZ/gZlv 

ZOOW~/v

ZOOZ/9Z/£ 

ZOOZlv~/£

ZOOZ/9Z/Z 

ZOOZlv~/Z

ZOOZH£H 

ZOOZ/HH 

aa

N'" N '"a a 
AmmSulfate 24-hour Concentration (ug/m 3 ) 

o o -" N N eN 

o 01 6 01 o 01 o 

4/11/2003 _ 

4/25/20031: 

5/9120031- -~

5/23/2003 ......- • 

6/612003 i 

I» 

..... 

-» 

5.3

;'3 

0 

G) ~. I» c: ... ., 3 Q. 

(I) CJ> 

./ c: 

G,ii;'

., ..... 6/20/2003 -I ./ I» (I) 

C 

Q.I» 7/412003 i ./ (")::

1»'< 

~(") 7/18/2003 i .-.. '<0 

o 

0 

8/1/2003i / 	 -'(1) 

-< ..... 8/15/2003 i (I) OJ 

I» ::::!: 

----- f + ., 0 

~ ~

8/29/2003~ \) .......... s:: ~~
 

- CD iii °Ul III c- o 

- VI 

9/12/20031- c UCD 

(,J 

iii - c. c- » 

9/26/2003 i / -........ » 

3 3

33 

(j) (j) 

1011 0/2003 	 c ---- ::;; ;;

c 

III 

CD 

10/24/2003""----"-J _ ..- c

CD 

C 

<0 <0 

3" 3"

1117120031 - ;S 

11/21/2003 

1215/2003 

12119/2003 
SELECTED DATES FOR BENCHMARKING ANALYSIS 

INDIAN GARDEN 

Predicted Measured 

Date AmmSulfate AmmSulfate 

(ulV'm-) (ulV'm3 ) 

8/26/2001 0.605 0.895 

2/4/2002 0.228 0.518 

11119/2002 0.224 0.501 

12/4/2002 0.235 0.742 

1/18/2003 0.221 0.275 

8/25/2003 0.373 0.992 

10/21/2003 0.355 0.914